1. Field of the Invention
The present invention relates to an endoscope system, and, more particularly, to an endoscope system using a solid state imaging device.
2. Description of the Prior Art
In order to observe and record an image of the inside of a cavity or opening of a living body or machinery, a fiberscope was conventionally used. A fiberscope is an endoscope having a viewing head unit, which is to be inserted into an opening of an object of interest, and includes at least an imaging optical system for picking up an image of the inside of the opening, a distal end portion of an image guide including a band of optical fiber for conveying an optical image formed by the imaging optical system to a proximal end thereof, and a distal end portion of a light guide including a band of optical fiber for conveying light to illuminate the inside of the opening. The optical image transmitted over the image guide was observed through a loupe or magnifying glass, photographed by a still camera, or monitored by a television system.
Recent progress in semiconductor technology has brought into use a self-scanning solid state imaging device, such as a charge-coupled device (CCD), for example, in the field of television cameras. In comparison with an image pickup tube, such as a vidicon, included in a conventional television camera, an imaging system using a solid state image sensor may be compactly designed. Thus proposals were made to install such a solid state image sensor in a head unit of an endoscope to convert an optical image of an object into electrical signals, which in turn are transmitted to a cathode-ray tube (CRT) display to be visualized thereon as a television picture. See Japanese Patent Laid-Open Publication No. 65962/1976, for example.
In order to monitor on a CRT a color television picture from an endoscope head unit, the following three techniques may be proposed. First, a fundamental technique is directed to a system including three separate solid state image sensors each for picking up an image of one of the three primary colors, red (R), green (G) and blue (B), formed and separated by an imaging optical system. A second system includes a single solid state imaging device having an array of photosensitive cells, over which is provided is a multi-color filter having color filter segments of the three primary colors, R, G and B, arrayed in association with the photosensitive cells in a mosaic pattern. In a third system, taking account of the specific structural features and of an endoscope and the situations of its use, the inside of a cavity of an object is illuminated with light which is transmitted over a light guide and changes in color cyclically between R, G, and B, so that a single imaging device picks up in time-serial object images of the three primary colors, R, G and B. The illuminating light is produced at the proximal end of the light guide by rotating in front of a light source a disk of color filter having three segments of the primary colors, R, G and B at a predetermined revolutionary speed.
Those systems, however, raise problems in applications of an endoscope. The above first system is fundamental and primitive system, and produces images of satisfactory image quality. It is, however, not possible to provide a small viewing head of an endoscope with such three image sensor systems, which would, if provided, cause the viewing head to be enlarged in size so as not to be insertable into a small cavity as of a living body, and therefore reduces the utility of the endoscope.
In the case of the second system, it is possible to design a head unit of an endoscope in small size. An image picked up by the single image sensor is, however, degraded in resolving power due to a lower number of green picture elements (pixels or pels), which are predominant over the resolving power of the picture, involved in the mosaic color filter having R, G and B elements on the image sensor. Particularly, with a smaller head unit, the imaging device does not include so many photosensitive cells on its imaging surface or array as to maintain a satisfactory image quality. Degradation in resolving power is critical in the case of a small viewing head.
In the aforementioned third system, a sole image sensor is operative in a field-serial fashion to produce three primary-color video signal components in time-serial, so that failure may occur in registration between fields of the three primary colors in the case of an object moving or changing faster. This may also cause degradation in image quality.